FROM THE MAINLAND OF SOUTH VICTORIA LAND 



221 



In a specimen from one portion of the dyke where the base appears to be a devitrified 

 glass, there appear numerous polygonal, nearly spherical, clear patches of aggregates of 

 a highly refracting colourless mineral suggesting change products of leucite. These 

 are as much as 0"25 mm. diameter. 



An analysis made by A. B. Walkom, B.Sc, of the dense variety of this rock first 

 mentioned is as follows : 



COMPOSITION OF THE NORM 



100-50 



100-54 



Chemico-mineralogical Classification: Class 2, Order 5, Bang 3, Sub-rang 4 (Andose). 



A comparison of this analysis with that of the granite shows a marked increase in 

 the basic constituents, lime and magnesia, and a concentration of soda as opposed to 

 potash. In the mineral components the fundamental difference lies in the elimination 

 of orthoclase with substitution of plagioclase. It is obviously a lamprophyric separation 

 from the granitic magma. 



The character of some of the occurrences are suggestive of tinguaites, but in no 

 case have staining methods revealed the presence of any felspathoid. Furthermore, 

 the alkali content of the above analysis is too low to suspect the presence of fel- 

 spathoids. 



These lamprophyres frequently resemble camptonites and it is probable that the 

 camptonites mentioned by Dr. Prior * are closely related to these intrusions. The 

 analysis of the rock type here described is, however, unlike that of the camptonites, for 

 it lies with the more acid lamprophyres. 



As hornblende is the dominant coloured constituent in all the outcrops, the choice 

 lies with kersanite and spessartite. The low magnesium and iron content and 

 the high proportion of plagioclase felspar clearly indicate the rock analysed to be a 

 kersantite. As hornblende is present in quantity in all the outcrops, sometimes to the 

 exclusion of the biotite, they are more correctly described as hornblende kersantites. 



Dr. Prior f has described a kersantite from a dyke intersecting the ancient crystalline 

 limestone formation in the Ferrar Glacier region. This is an augite-biotite-kersantite 

 and differs somewhat in appearance from the dyke rocks commonly met with by us. 

 There is, nevertheless, no doubt but that it is genetically related to that just described. 

 The analysis is strikingly similar. 



hoc. cit., p. 129. 



f Loc. cit., p. 130. 



